ESTRO 2024 - Abstract Book

S4219

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2024

The impact of motion on onboard MRI-guided pencil beam scanned proton therapy treatments

Alisha Duetschler 1,2 , Sairos Safai 1 , Damien C Weber 1,3,4 , Antony J Lomax 1,2 , Ye Zhang 1

1 Paul Scherrer Institute, Center for Proton Therapy, Villigen, Switzerland. 2 ETH Zürich, Department of Physics, Zürich, Switzerland. 3 University Hospital of Zürich, Department of Radiation Oncology, Zürich, Switzerland. 4 Inselspital, Bern University Hospital, University of Bern, Department of Radiation Oncology, Bern, Switzerland

Purpose/Objective:

The precision and efficacy of pencil beam scanned (PBS) proton therapy treatments present notable challenges, especially for tumour sites experiencing anatomical alterations. Onboard magnetic resonance imaging (MRI) guidance has emerged as a pivotal approach to enhance PBS proton treatments by directly tracking tumours based on the potential acquired high-contrast MR images in real time. Consequently, the computation of dose distributions under the influence of magnetic fields and the subsequent optimisation of the plan in a 4D framework becomes imperative. This study aims to investigate, for the first time as far as we know, the dosimetric effects of respiratory motion for 4D treatments in the context of onboard MRI-guided proton therapy treatment plans. Moreover, the findings are compared to the scenarios without a magnetic field, typically in conventional settings.

Material/Methods:

To facilitate 4D dose calculations in magnetic fields, we extended the previously established analytical dose calculation algorithm that accounts for proton beam deflection in perpendicular magnetic fields [1] by further considering both deformable motion and motion-induced density changes. Utilising two geometric wedge phantoms alongside three liver and two lung patient cases, we optimised static treatment plans both with and without the presence of magnetic field effects. Specifically, plans were optimised for 0, 0.5 and 1.5 T and also with gantry angle adjustments (0.5 T +5° and 1.5 T +15°) to match similar beam trajectories within the patient compared to the 0 T reference plans. Subsequently, we incorporated the impact of motion using 4D dose evaluations, considering motion data from multiple 4DCT(MRI) datasets capturing varied breathing cycles for liver and lung (10/15 cycles respectively) [2, 3]. These 4D dose calculations were performed without motion mitigation and with 8-times volumetric rescanning (VS8). Different starting phases were assessed for each 4D dose, with the analysis focusing on the CTV dose coverage V95% and the homogeneity measure D5%-D95%.

Results:

A comparable motion effect was observed independent of the magnetic field strength for the two geometrical phantoms simulated with rigid motion perpendicular to the beam and parallel to the magnetic field. Dose distributions for the more heterogeneous geometrical phantom and a selected lung case can be found in Figure 1. Likewise, for the five 4DCT(MRI) cases, the influence of motion remained consistent across all magnetic field strengths, both with and without gantry angle adjustments. Figure 2 illustrates the dosimetric impact on the CTV coverage V95% and homogeneity D5%-D95%. Averaged over all five cases, the reduction in CTV V95% due to motion, compared to the static plans, was marginally more minor in the presence of a magnetic field (17% (1.5 T) and 19% (0.5 T) compared to 20% for the 0 T reference plan).